LLE Review 151

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• Review 151 Editor

Highlights

This volume of the LLE Review, covering April–June 2017, features “Flying Focus: Spatiotemporal Control of the Laser Focus,” which provides a brief overview of an advanced focusing scheme that enables a small-diameter laser focus to propagate nearly 100× its Rayleigh length. The technique provides a novel control over laser–plasma interactions that removes the need for long-focal-length systems or guiding structures to maintain high intensities over long distances and decouples the velocity of the focal spot from the group velocity of the light.

Additional highlights of research presented in this issue include the following:

• An accompanying article proposes a new laser-amplifier scheme utilizing stimulated Raman scattering in plasma in conjunction with a “flying focus” to generate a chromatic focusing system combined with a chirped pump beam that provides spatiotemporal control over the pump focal spot.
• The impact of beam speckle and polarization smoothing on cross-beam energy transfer (CBET) are compared using the 3-D wave-based laser–plasma interaction code LPSE and ray-based models. A model for CBET between linearly polarized speckled beams is presented that uses ray tracing to solve for the real speckle pattern of the unperturbed laser beams within the eikonal approximation that gives excellent agreement with wave-based calculations.
• A report on planar laser–plasma interaction (LPI) experiments at the National Ignition Facility in the regimes of electron density scale length (500 to 700 µm), electron temperature (3 to 5 keV), and laser intensity (6 to 16× 10¹⁴ W/cm²) that have significant implications for the mitigation of LPI hot-electron preheat in direct-drive ignition designs is presented.
• An effective method for determining the offsets of the cryogenic implosion cores generated in OMEGA’s inertial confinement fusion experiments is demonstrated. The method utilizes images taken by the gated microscopic x-ray imaging (GMXI) diagnostic module. The cryogenic shots images are cross correlated onto images of their respective pulse shape setup (PSS) shots images.
• Tomographic x-ray images of targets imploded in direct drive configuration on the 60-beam OMEGA laser to measure their 3-D drive asymmetry in target modes ℓ = 1, ℓ = 2, and ℓ = 3 at a convergence ratio of ~3 are investigated. The analysis provides a means for determining the residual target modes and the laser modes that compensate them that agree with 3-D simulations that predict significant enhancements in fusion performance.
• Thermal contraction anomalies in glow-discharge polymer (GDP) capsules with a layer of an equimolar mixture of deuterium and tritium (DT) that do not exhibit expected contraction compared to GDP with only deuterium and polystyrene capsules permeated with only DT are discussed. It is speculated the highly cross-linked GDP shell is under compressive stress after fabrication and experiences bond breakage when exposed to high-density DT during permeation.